In animals, the brain is the control center of the central nervous system, responsible for behavior. The brain is located in the head, protected by the skull and close to the primary sensory apparatus of vision, hearing, equilibrioception (balance), sense of taste, and olfaction (smell). While all vertebrates have a brain, most invertebrates have either a centralized brain or collections of individual ganglia. Some animals such as cnidarians and echinoderms do not have a centralized brain, instead have a decentralized nervous system, while animals such as sponges lack both a brain and nervous system entirely. Brains can be extremely complex. For example, the human brain contains roughly 23 billion neurons, each linked to as many as 10,000 other neurons.

Overview

The brain is the central information-processing organ of the body. It innervates the head through cranial nerves, and it communicates with the spinal cord, which innervates the body through spinal nerves. Nervous fibers transmitting signals from the brain are called efferent fibers. The fibers transmitting signals to the brain are called afferent fibers (or sensory fibers). Nerves can be afferent, efferent or mixed (i.e., containing both types of fibers).

The brain undergoes transitions from wakefulness to sleep (and subtypes of these states). These state transitions are crucially important for proper brain functioning. (For example, it is believed that sleep is important for knowledge consolidation, as the neurons appear to organize the day's stimuli during deep sleep by randomly firing off the most recently used neuron pathways; additionally, without sleep, normal subjects are observed to develop symptoms resembling mental illness, even auditory hallucinations). Every brain state is associated with characteristic brain waves.

Neurons are electrically active brain cells that process information, whereas Glial cells perform supporting function. In addition to being electrically active, neurons constantly synthesize neurotransmitters. Neurons modify their properties (guided by gene expression) under the influence of their input signals. This plasticity underlies learning and adaptation. It is notable that some unused neuron pathways (constructions which have become physically isolated from other cells) may continue to exist long after the memory is absent from consciousness, possibly developing the subconscious.

The study of the brain is known as neuroscience, a field of biology aimed at understanding the functions of the brain at every level, from the molecular up to the psychological. There is also a branch of psychology that deals with the anatomy and physiology of the brain, known as biological psychology. This field of study focuses on each individual part of the brain and how it affects behavior.

History

Early views on the function of the brain regarded it as little more than cranial stuffing. In Ancient Egypt, from the late Middle Kingdom onwards, in preparation for mummification, the brain was regularly removed, for it was the heart that was assumed to be the seat of intelligence. According to Herodotus, during the first step of mummification, "The most perfect practice is to extract as much of the brain as possible with an iron hook, and what the hook cannot reach is mixed with drugs." Over the next five-thousand years, this view came to be reversed; the brain is now known to be seat of intelligence, although idiomatic variations of the former remain, as in memorizing something "by heart".[1]

The first thoughts on the field of psychology came from ancient philosophers, such as Aristotle. As thinkers became more in tune with biomedical research over time, as was the case with medieval psychologists such as Alhazen and Avicenna for example, the concepts of experimental psychology and clinical psychology began emerging. From that point, different branches of psychology emerged with different individuals creating new ideas, with modern psychologists such as Freud and Jung contributing to the field.

Mind and brain

The distinction between the mind and the brain is fundamental in philosophy of mind. The mind-body problem is one of the central problems in the history of philosophy. The brain is the physical and biological matter contained within the skull, responsible for electrochemical neuronal processes. The mind, in contrast, consists in mental attributes, such as beliefs, desires, perceptions, and so on. There are scientifically demonstrable correlations between mental events and neuronal events; the philosophical question is whether these phenomena are identical or are related in some other way.

The philosophical positions on the mind-body problem fall into two main categories. The first category is dualism, according to which the mind exists independently of the brain. Dualist theories are further divided into substance dualism and property dualism. Descartes is perhaps the most prominent substance dualist, while property dualism is more popular among contemporary dualists like David Chalmers. The second category is materialism, according to which mental phenomena are identical to neuronal phenomena. A third category of view, idealism, claims that only mental substances and phenomena exist. This view, most prominently held by 18th century English philosopher George Berkeley, has few contemporary adherents.

Both dualism and materialism face serious philosophical challenges. Dualism requires that we admit non-physical substances or properties into our ontology, a move that places dualism in apparent conflict with the scientific world view. Materialism, on the other hand, must provide an explanation of how two seemingly different kinds of phenomena - the mental and the physical - could be identical. This challenge can be seen by noting that mental phenomena have certain characteristics - particularly intentionality and phenomenal character - that physical phenomena do not, and seemingly could not, have.

Comparative anatomy

A mouse brain.

Three groups of animals have notably complex brains: the arthropods (insects, crustaceans, arachnids, and others), the cephalopods (octopuses, squids, and similar mollusks), and the craniates (vertebrates and hagfish).[2] The brain of arthropods and cephalopods arises from twin parallel nerve cords that extend through the body of the animal. Arthropods have a central brain with three divisions and large optical lobes behind each eye for visual processing.[2]

Mammals have a six-layered neocortex (or homotypic cortex, neopallium), in addition to having some parts of the brain that are allocortex.[3] In mammals, increasing convolutions of the brain are characteristic of animals with more advanced brains. These convolutions provide a larger surface area for a greater number of neurons while keeping the volume of the brain compact enough to fit inside the skull. The folding allows more grey matter to fit into a smaller volume, similar to a really long slinky being able to fit into a tiny box when completely pushed together. The folds are called gyri, while the spaces between the folds are called sulci.

In birds, the part of the brain that functionally corresponds to the neocortex is called nidopallium and derives from a different part of the brain. Some birds (like corvids and parrots) have intelligence equal to great apes, but even in these, the brain region that forms the mammalian neocortex is in fact almost entirely absent.

Although the general histology of the brain is similar from person to person, the structural anatomy can differ. Apart from the gross embryological divisions of the brain, the location of specific gyri and sulci, primary sensory regions, and other structures differs between species.

Mammals and other vertebrates

The telencephalon (cerebrum) is the largest region of the mammalian brain. This is the structure that is most easily visible in brain specimens, and is what most people associate with the "brain". In humans and several other animals, the fissures (sulci) and convolutions (gyri) give the brain a wrinkled appearance. In non-mammalian vertebrates with no cerebrum, the metencephalon is the highest center in the brain. Because humans walk upright, there is a flexure, or bend, in the brain between the brain stem and the cerebrum. Other vertebrates do not have this flexure. Generally, comparing the locations of certain brain structures between humans and other vertebrates often reveals a number of differences.

Behind (or in humans, below) the cerebrum is the cerebellum. The cerebellum is known to be involved in the control of movement,[3] and is connected by thick white matter fibers (cerebellar peduncles) to the pons.[4] The cerebrum has two cerebral hemispheres. The cerebellum also has hemispheres. The telencephalic hemispheres are connected by the corpus callosum, another large white matter tract. An outgrowth of the telencephalon called the olfactory bulb is a major structure in many animals, but in humans and other primates it is relatively small.

Vertebrate nervous systems are distinguished by bilaterally symmetricalencephalization. Encephalization refers to the tendency for more complex organisms to gain larger brains through evolutionary time. Larger vertebrates develop a complex, layered and interconnected neuronal circuitry. In modern species most closely related to the first vertebrates, brains are covered with gray matter that has a three-layer structure (allocortex). Their brains also contain deep brain nuclei and fiber tracts forming the white matter. Most regions of the human cerebral cortex have six layers of neurons (neocortex).[4]

In recent years it was realized that certain birds have developed high intelligence entirely convergently from mammals such as humans. Hence, the functional areas of the avian brain have been redefined by the Avian Brain Nomenclature Consortium.

Humans

The structure of the human brain differs from that of other animals in several important ways. These differences allow for many abilities over and above those of other animals, such as advanced cognitive skills. Human encephalization is especially pronounced in the neocortex, the most complex part of the cerebral cortex. The proportion of the human brain that is devoted to the neocortex—especially to the prefrontal cortex—is larger than in all other mammals (indeed larger than in all animals, although only in mammals has the neocortex evolved to fulfill this kind of function).

Humans have unique neural capacities, but much of their brain structure is similar to that of other mammals. Basic systems that alert the nervous system to stimulus, that sense events in the environment, and monitor the condition of the body are similar to those of even non-mammalian vertebrates. The neural circuitry underlying human consciousness includes both the advanced neocortex and prototypical structures of the brainstem. The human brain also has a massive number of synaptic connections allowing for a great deal of parallel processing.

Neurobiology

The brain is composed of two broad classes of cells, neurons and glia, both of which contain several different cell types which perform different functions. Interconnected neurons form neural networks (or neural ensembles). These networks are similar to man-made electrical circuits in that they contain circuit elements (neurons) connected by biological wires (nerve fibers). These do not form simple one-to-one electrical circuits like many man-made circuits, however. Typically neurons connect to at least a thousand other neurons.[5] These highly specialized circuits make up systems which are the basis of perception, different types of action, and higher cognitive function.

Structure

Neurons are the cells that generate action potentials and convey information to other cells; these constitute the essential class of brain cells.

In addition to neurons, the brain contains glial cells in a roughly 10:1 proportion to neurons. Glial cells ("glia" is Greek for “glue”) form a support system for neurons. They create the insulating myelin, provide structure to the neuronal network, manage waste, and clean up neurotransmitters. Most types of glia in the brain are present in the entire nervous system. Exceptions include the oligodendrocytes which myelinate neural axons (a role performed by Schwann cells in the peripheral nervous system). The myelin in the oligodendrocytes insulates the axons of some neurons. White matter in the brain is myelinated neurons, while grey matter contains mostly cell soma, dendrites, and unmyelinated portions of axons and glia. The space between neurons is filled with dendrites as well as unmyelinated segments of axons; this area is referred to as the neuropil.

In mammals, the brain is surrounded by connective tissues called the meninges, a system of membranes that separate the skull from the brain. This three-layered covering is composed of (from the outside in) the dura mater, arachnoid mater, and pia mater. The arachnoid and pia are physically connected and thus often considered as a single layer, the pia-arachnoid. Below the arachnoid is the subarachnoid space which contains cerebrospinal fluid, a substance that protects the nervous system. Blood vessels enter the central nervous system through the perivascular space above the pia mater. The cells in the blood vessel walls are joined tightly, forming the blood-brain barrier which protects the brain from toxins that might enter through the blood.

The brain is bathed in cerebrospinal fluid (CSF), which circulates between layers of the meninges and through cavities in the brain called ventricles. It is important both chemically for metabolism and mechanically for shock-prevention. For example, the human brain weighs about 1-1.5 kg or about 2-3 lb. The mass and density of the brain are such that it will begin to collapse under its own weight if unsupported by the CSF. The CSF allows the brain to float, easing the physical stress caused by the brain’s mass.

Function

Vertebrate brains receive signals through nerves arriving from the sensors of the organism. These signals are then processed throughout the central nervous system; reactions are formulated based upon reflex and learned experiences. A similarly extensive nerve network delivers signals from a brain to control important muscles throughout the body. Anatomically, the majority of afferent and efferent nerves (with the exception of the cranial nerves) are connected to the spinal cord, which then transfers the signals to and from the brain.

To control movement the brain has several parallel systems of muscle control. The motor system controls voluntary muscle movement, aided by the motor cortex, cerebellum, and the basal ganglia. The system eventually projects to the spinal cord and then out to the muscle effectors. Nuclei in the brain stem control many involuntary muscle functions such as heart rate and breathing. In addition, many automatic acts (simple reflexes, locomotion) can be controlled by the spinal cord alone.

Brains also produce a portion of the body's hormones that can influence organs and glands elsewhere in a body—conversely, brains also react to hormones produced elsewhere in the body. In mammals, the hormones that regulate hormone production throughout the body are produced in the brain by the structure called the pituitary gland.

Evidence strongly suggests that developed brains derive consciousness from the complex interactions between the numerous systems within the brain. Cognitive processing in mammals occurs in the cerebral cortex but relies on midbrain and limbic functions as well. Among "younger" (in an evolutionary sense) vertebrates, advanced processing involves progressively rostral (forward) regions of the brain.

Hormones, incoming sensory information, and cognitive processing performed by the brain determine the brain state. Stimulus from any source can trigger a general arousal process that focuses cortical operations to processing of the new information. This focusing of cognition is known as attention. Cognitive priorities are constantly shifted by a variety of factors such as hunger, fatigue, belief, unfamiliar information, or threat. The simplest dichotomy related to the processing of threats is the fight-or-flight response mediated by the amygdala and other limbic structures.

Diseases may affect specific neurotransmitter systems. For example, Parkinson's disease is at least in part related to failure of dopaminergic cells in deep-brain nuclei, for example the substantia nigra. Treatments potentiating the effect of dopamine precursors have been proposed and effected, with moderate success.

Pathology

Clinically, death is defined as an absence of brain activity as measured by EEG. Injuries to the brain tend to affect large areas of the organ, sometimes causing major deficits in intelligence, memory, and movement. Head trauma caused, for example, by vehicle and industrial accidents, is a leading cause of death in youth and middle age. In many cases, more damage is caused by resultant edema than by the impact itself. Stroke, caused by the blockage or rupturing of blood vessels in the brain, is another major cause of death from brain damage.

Methods of observation

Each method for observing activity in the brain has its advantages and drawbacks.

Electrophysiology

Electrophysiology allows scientists to record the electrical activity of individual neurons or groups of neurons.

EEG

By placing electrodes on the scalp one can record the summed electrical activity of the cortex in a technique known as electroencephalography (EEG). EEG measures the mass changes in electrical current from the cerebral cortex, but can only detect changes over large areas of the brain with very little sub-cortical activity.

MEG

Apart from measuring the electric field around the skull it is possible to measure the magnetic field directly in a technique known as magnetoencephalography (MEG). This technique has the same temporal resolution as EEG but much better spatial resolution, although admittedly not as good as fMRI. The main advantage over fMRI is a direct relationship between neural activation and measurement.

fMRI and PET

A scan of the brain using fMRI

Functional magnetic resonance imaging (fMRI) measures changes in blood flow in the brain, but the activity of neurons is not directly measured, nor can it be distinguished whether this activity is inhibitory or excitatory. fMRI is a noninvasive, indirect method for measuring neural activity that is based on BOLD; Blood Oxygen Level Dependent changes. The changes in blood flow that occur in capillary beds in specific regions of the brain are thought to represent various neuronal activities (metabolism of synaptic reuptake). Similarly, a positron emission tomography (PET), is able to monitor glucose and oxygen metabolism as well as neurotransmitter activity in different areas within the brain which can be correlated to the level of activity in that region.

Behavioral

Behavioral tests can measure symptoms of disease and mental performance, but can only provide indirect measurements of brain function and may not be practical in all animals. In humans however, a neurological exam can be done to determine the location of any trauma, lesion, or tumor within the brain, brain stem, or spinal cord.

Anatomical

Autopsy analysis of the brain allows for the study of anatomy and protein expression patterns, but is only possible after the human or animal is dead. Magnetic resonance imaging (MRI) can be used to study the anatomy of a living creature and is widely used in both research and medicine.

Creating algorithms to mimic a biological brain is very difficult because the brain is not a static arrangement of circuits, but a network of vastly interconnected neurons that are constantly changing their connectivity and sensitivity. More recent work in both neuroscience and artificial intelligence models the brain using the mathematical tools of chaos theory and dynamical systems. Current research has also focused on recreating the neural structure of the brain with the aim of producing human-like cognition and artificial intelligence.

As nourishment

Like most other internal organs, the brain can serve as nourishment. For example, in the Southern United States canned pork brain in gravy can be purchased for consumption as food. This form of brain is often fried with scrambled eggs to produce the famous "Eggs n' Brains".[10] The brain of animals also features in French cuisine such as in the dish tête de veau, or head of calf. Although it might consist only of the outer meat of the skull and jaw, the full meal includes the brain, tongue, and glands. Similar delicacies from around the world include Mexican tacos de sesos made with cattle brain as well as squirrel brain in the US South.[11] The Anyang tribe of Cameroon practiced a tradition in which a new tribal chief would consume the brain of a hunted gorilla while another senior member of the tribe would eat the heart.[12] Indonesian cuisine specialty in Minangkabau cuisine also served beef brain in a gravy coconut milk named gulai otak (beef brain curry). Roasted or fried goat brain is eaten in south India and some parts of north India.

Consuming the brain and other nerve tissue of animals is not without risks. The first problem is that the makeup of the brain is 60% fat due to the myelin (which itself is 70% fat) insulating the axons of neurons and glia.[13] As an example, a 140 g can of "pork brains in milk gravy", a single serving, contains 3500 milligrams of cholesterol, 1170% of our recommended daily intake.[14]

Brain consumption can result in contracting fatal transmissible spongiform encephalopathies such as Variant Creutzfeldt-Jakob disease and other prion diseases in humans and mad cow disease in cattle.[15] Another prion disease called kuru has been traced to a funerary ritual among the Fore people of Papua New Guinea in which those close to the dead would eat the brain of the deceased to create a sense of immortality.[16] Some archaeological evidence suggests that the mourning rituals of European Neanderthals also involved the consumption of the brain.[17] Because of the risk of being infected by prions one should always wear gloves when handling brains.

It is also well known in the hunting community that the brain of wild animals should not be consumed, due to the risk of chronic wasting disease. The brain is still useful to hunters, in that most animals have enough brain matter for use in the tanning of their own hides.

Brain energy consumption

The neurons of the brain require a lot of energy. Although the brain represents only 2% of the body weight, it receives 15% of the cardiac output, 20% of total body oxygen consumption, and 25% of total body glucose utilization. The energy consumption for the brain to simply survive is 0.1 calories per minute, while this value can be as high as 1.5 calories per minute during crossword puzzle-solving.[18] The demands of the brain limit its size in many species. Molossid bats and the Vespertilionid Nyctalus spp. have brains that have been reduced from the ancestral form to invest in wing-size for the sake of manoeuverability. This contrasts with fruit bats, which require more advanced neural structures and do not pursue their prey.[19]

References

↑Hendrickson, Robert (2000). The Facts on File Encyclopedia of Word and Phrase Origins. New York: Facts on File. ISBN978-0816040889. The ancient Greeks believed that the heart, the most noticeable internal organ, was the seat of intelligence and memory as well as emotion. This belief was passed on down the ages and became the basis for the English expression 'learn by heart,' which is used by Chaucer (1374) and must have been proverbial long before that. 'To record' reminds us again of this ancient belief in the heart as the seat of the mind. When writing wasn't a simple act, things had to be memorized; thus we have the word 'record,' formed from the Latin 're,' 'again,' and 'cor,' 'heart,' which means exactly the same as 'learn by heart.'Unknown parameter |month= ignored (help)

↑ 2.02.1Butler, Ann B. (2000). "Chordate Evolution and the Origin of Craniates: An Old Brain in a New Head". The Anatomical Record. 261: 111–125.